The invention relates generally to a method for preparing photodichroic alkali fluoride crystals and more particularly to additive and electrolytic coloration of alkali fluoride crystals.
Photodichroism is the property of absorbing wavelengths of light differently along each axis of the crystal. To impart this property in a crystal, the crystal is colored, i.e., has color centers introduced into its crystalline structure. The most common of these centers are the F, F.sub.A, M, and M.sub.A centers with the M-center being the most important.
Alkali halide crystals containing color centers such as the aforementioned ones have shown considerable promise as optical elements capable of reversible, on-line write-erase-read operations where the reading would be nondestructive. Information would be related to the presence or absence of dichroic absorption (i.e. -- differences in absorption of polarized light) and would be introduced into the crystal by causing M-center reorientations with polarized light.
The most promising alkali halide for possible use near room temperature is NaF. Unfortunately, this fluoride, like the other alkali fluorides, has been colored only through the use of ionizing radiation such as X-rays or high-energy electrons. But such a technique introduces interstitials which eventually recombine with the color centers and mutually annihilate. The crystals accordingly are less stable then crystals colored by other techniques.
Two methods of coloring alkali halide crystals which do not produce interstitials are the additive and electrolytic methods. However, attempts to produce well-characterized color centers in alkali fluoride crystals by these techniques have not been successful. The crystals are either not colored or contain metallic-colored particles with diameters as large as 30 nm. Such particles give rise to Tyndall scattering with an apparent absorption in the wavelength range from 505 to 570 nm.
It has been known that alkali earth metals cations cause metallic particles to form during coloration. Previous methods were highly successful in removing these cations along with other foreign cations except for Mg.sup.++ and Ca.sup.++. Also these methods of purifying alkali fluorides were not able to eliminate the fluorosilicate ion (SiF.sub.6.sup.=) below 10 ppmA and were extremely slow and inconsistent in removing the hydroxyl ion.
Throughout this disclosure concentrations are expressed in units of ppmA. Such units are synonymous with the units of moles of additive per 1,000,000 moles of crystal.